Method and device for separating a substrate

ABSTRACT

A method and device for separating a substrate with a laser beam. The duration of the laser beam&#39;s effect is extremely short, so the substrate is only modified concentrically about the laser beam axis (Z) without it degrading the substrate material. While the laser beam acts upon the substrate, the substrate moves relative to a laser machining head, producing plural filament-type modifications along a separating surface to be incorporated. The laser beam is initially diverted by a transmission medium having a higher intensity dependent refractive index than air, then reaches the substrate. The non-constant pulsed laser intensity increases to a maximum over the temporal course of the single pulse, then reduces, and the refractive index changes. The laser beam focus point moves between the substrate&#39;s outer surfaces along the beam axis (Z), reaching the desired modification along the beam axis (Z) without correcting the laser machining head in the z-axis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage application under 35 U.S.C.§371 of International Application No. PCT/DE2014/100119, filed on Apr.3, 2014, and claims benefit to German Patent Applications No. DE 10 2013103 370.9 filed on Apr. 4, 2013, and DE 10 2013 112 035.0, filed on Oct.31, 2013. The International Application was published in German on Oct.9, 2014, as WO 2014/161535 A2 under PCT Article 21(2).

FIELD

The invention relates to a method and a device for separating, inparticular breaking up, a substrate, in particular a glass substrate,which is usable for example as an interposer or microcomponent, using alaser beam.

BACKGROUND

A method of this type and a device for carrying out the separationprocess are used in practice for example for separating or splittingwafers, glass substrates and plates. Substrates of this type are alsoused for example as interposers for electrically connecting theterminals of a plurality of homogeneous or heterogeneous microchips.

In practice, separation by cutting is a critical step in the processingof wafers or glass substrates, which is typically based on the use ofdiamond cutting tools and carried out for example at a speed of 30 cm/sfor displays. However, the quality of the edges which can be achieved bythis process is unsatisfactory, and leads to significant drawbacks interms of the service life, quality and reliability of the product, butalso in the resulting cleaning costs.

In this context, it is found to be a challenge to process the substrateinto usable elements. The prior art has not yet addressed, inparticular, the economical production of the plurality of separatingfaces in a substrate for example in the production of wafers.

US 2013/126573 A1 discloses a separating method for producing asubstrate in which the substrate is irradiated with one or more pulsesof a focused laser beam. In this context, the substrate is transparentto the focused laser beam, whilst the laser pulses are selected in sucha way, in terms of the energy and pulse duration, that a duct-likefilament is produced within the substrate. By displacing the substraterelative to the focused laser beam, additional, spatially separatedfilaments are produced, which thus define a separating face. Thesubstrate consists for example of glass, crystal, quartz, diamond orsapphire. For a corresponding material thickness of the substrate, aplurality of focal points of the focused laser beam are selected in sucha way that filaments are produced in at least one of the two or morelayers. In this context, the filament produced by the focused laser beamin a first layer should propagate into at least one additional layer andproduce a second filament in this further layer. Further, it may also beprovided for a second beam focus to be produced in a second layer. Inthis method, the use of comparatively expensive femtosecond orpicosecond lasers and the complex configuration, in which a pulsesequence of individual pulses and of particular repetition rates of thepulse sequences in accordance with particular prescriptions is provided,are found to be disadvantageous. In particular, a time delay betweensuccessive pulses in the pulse sequence is smaller than a duration ofthe relaxation of a material modification.

The term “stealth dicing” refers to a laser machining method in which ina first step a laser beam acts on a layer within a substrate. In asecond step a tensile stress is applied so as to separate the substratealong the action points in the layer. This layer is an internal surfacein the wafer, which is modified by the laser within the substrate duringthe processing and forms the starting point for dividing the substrateduring the processing. The tensile stress subsequently brings about theseparation of the substrate into small portions.

A method of this type for separating a substrate, for example asemiconductor substrate in the production of a semiconductor componentor the like, is known for example from U.S. Pat. No. 8,518,800 B2. Inthis context, the substrate is irradiated with laser light in such a waythat a multiphoton absorption phenomenon is produced within thesubstrate, whereby a light convergence point and thus a modified areaare formed within the substrate. By forming a cutting onset point regionwithin the substrate, a break is produced in the substrate in thedirection of the thickness extent thereof, without external action orwhilst exerting a force, starting from the cutting onset point regionwhich acts as the starting point.

EP 2 503 859 A1 further discloses a method in which a glass substrate isprovided with through-holes, the glass substrate consisting of aninsulator such as glass, for example silicate glass, sapphire, plasticsmaterial or ceramic and semiconductors such as silicon. The glasssubstrate is irradiated using a laser, for example a femtosecond laser,which is focused on a focal point at a desired position within the glasssubstrate. The through-holes are produced by a method in which theregions of the glass substrate which have been modified by the laser aredipped in an etching solution and the modified regions are thus removedfrom the glass substrate. This etching makes use of the effect wherebythe modified region is etched extremely rapidly by comparison with theunmodified regions of the glass substrate. Blind holes orthrough-openings can be produced in this manner. A copper solution issuitable for filling the through-opening. To achieve a desired deptheffect, in other words a through-hole between the outer substrate faces,the focal point has to be displaced during continuous irradiation, inother words tracked in the direction of the z-axis.

More generally, the combination of selective laser treatment with asubsequent etching process in the form of selective laser-inducedetching is also known as ISLE (in-volume selective laser-inducedetching).

DE 10 2010 025 966 B4 further discloses a method in which in a firststep focused laser pulses are directed onto the glass substrate, theradiation intensity of said pulses being high enough to result in localathermal decomposition along a filament-like track in the glass. In asecond method step, the filament-like tracks are expanded into holes bysupplying high-voltage power to opposing electrodes, resulting indielectric breakdowns through the glass substrate along thefilament-like tracks. These breakdowns expand under electrothermalheating and evaporation of hole material, until the process is halted byswitching off the power supply upon achieving the desired hole diameter.Alternatively or in addition, the tracks may also be expanded usingreactive gases, which are directed onto the hole sites using nozzles.The through-opening sites may also be expanded using supplied etchinggas. The comparatively complex process, resulting from the fact theglass substrate initially has to be broken through by the athermaldecomposition and the diameter of the filament-like tracks has to beexpanded into holes in the following step, has proved to bedisadvantageous.

Further, U.S. Pat. No. 6,400,172 B1 discloses the introduction ofthrough-openings in semiconductor materials by laser.

SUMMARY

An aspect of the invention provides a method for separating a substrateusing an optical system, a thickness of the substrate not exceeding 2 mmin a region of a separating line, the method comprising: applying pulsedlaser radiation having a pulse duration (t) to a substrate material ofthe substrate, which material is transparent at least in part to a laserwavelength, the laser radiation being focused using the optical systemhaving an original focal depth (f1), an intensity of the laser radiationleading to a modification of the substrate along a beam axis (Z) of thelaser radiation, but not to material removal which goes all the waythrough, and the pulsed laser radiation being moved along any desiredseparating line parallel to the primary extension plane of thesubstrate, bringing about a subsequent separation process along theseparating line, wherein the pulsed laser radiation is focused by thesame optical system, which is unchanged per se, at a focal depth (f2)different from the original focal depth (f1), by non-linearself-focusing within the pulse duration (t) of an individual pulse (P).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in even greater detail belowbased on the exemplary figures. The invention is not limited to theexemplary embodiments. All features described and/or illustrated hereincan be used alone or combined in different combinations in embodimentsof the invention. The features and advantages of various embodiments ofthe present invention will become apparent by reading the followingdetailed description with reference to the attached drawings whichillustrate the following:

FIG. 1 is a schematic drawing of a prior art method;

FIG. 2 is a flow chart comprising a plurality of method steps forintroducing a plurality of through-openings into a substrate;

FIG. 3 shows the intensity-dependent focal point during an individualpulse; and

FIG. 4 is a graph showing the intensity distribution over time duringthe duration of an individual pulse.

DETAILED DESCRIPTION

An aspect of the invention provides an option for substantiallysimplifying a method and a device for separating a substrate, and inparticular for reducing the time taken to carry this out.

An aspect of the invention thus provides a method in which the laserradiation is focused by non-linear self-focusing within the pulseduration of a single pulse by the unchanged optical system using a focaldepth differing from the original focal depth. The invention makes useof the fact that the intensity of a pulsed laser is not constant for anindividual pulse, but rather has an intensity which increases to amaximum and subsequently falls away over the temporal progression of theindividual pulse. Because the refractive index also increases to amaximum as a result of the increasing intensity, in a mannercorresponding to a normal distribution over the temporal progression foran individual pulse, the focal depth of the optical system, in otherwords the distance from a laser machining head or the lens, changes,independently of the geometric focal point determined by the focusingoptics.

This effect of non-linear self-focusing is made use of in that thedistance between the focal points between a maximum and a minimumintensity at least corresponds to the desired longitudinal extent, inother words to the thickness in the region of a separating line. In asurprisingly simple manner, this results in a spatial displacement inthe direction of the beam axis during the duration of an individualpulse, which leads to the desired modification in the region of theentire primary extension, in the direction of the beam axis. Tracking ofthe focal point, which is unavoidable in the prior art, can be omittedin this case. Thus, in particular, no control system for moving thelaser focus through the substrate is required. In this way, a modifiedregion of the substrate is produced along the separating line as aseparating face or predetermined breaking face. Thus, according to theinvention, not only is the control system outlay required for thispurpose omitted, but the machining duration can also be considerablyreduced, for example to the duration of an individual pulse. Thenon-linear refractive index of the transmissive medium is a linearfunction of the intensity, and so the selection of a suitable materialand suitable dimensions is dependent on the intensity of the laserradiation used.

In this context, the laser beam is directed onto the substratesufficiently briefly that the substrate is merely modified along a beamaxis of the laser beam, without destruction which penetrates through thesubstrate occurring, anisotropic material removal for example beingcarried out in the next step in the regions of the substrate which havepreviously undergone modification by means of the laser beam, so as thusto carry out the separation optionally in connection with an assistingexternal force action.

The laser power input is used to excite or trigger a reaction and amodification by conversion, the effect of which is only made use of foror only leads to the desired material removal in the subsequent methodstep.

Because the separation process, on the basis of the modification andoptionally subsequent anisotropic material removal, is carried out by anetching method, it is possible to use a planar-action removal method,which only places very low requirements on the process, rather than asequential one for the separation process. Instead, over the duration ofaction, the material duration can be carried out quantitatively andqualitatively for all regions which are pre-treated in the describedmanner and correspondingly modified, reducing the expenditure of timefor producing the plurality of recesses or through-openings considerablyoverall.

The focal point at minimum intensity may be directed onto an outersurface of the substrate. However, it has already been found to beparticularly promising if the laser radiation is focused onto a remoteside of the substrate at a distance therefrom, in such a way that thefocal point of the laser radiation is set so as to be positioned on arear side, remote from the laser radiation, at a distance from thesurface of the substrate. As a result, the laser beam is initiallydirected onto a focal point positioned outside the substrate. Therefractive index which changes as a result of the increasing intensitysubsequently leads to a spatial displacement of the focal point throughthe substrate along the beam axis. This ensures that a sufficiently highintensity for producing the modification is applied to every focal pointwithin the substrate.

The duration of the beam action may, of course, comprise a plurality ofpulse durations for an unchanged relative position of the lasermachining head with respect to the substrate, for example so as furtherto optimise the modification of the substrate material. However, it isparticularly advantageous if the laser beam is deflected onto each focalpoint for the duration of a single pulse. In this way, the previous andsubsequent pulses of the laser beam are directed onto positions spacedapart in the plane of the substrate, in such a way that adjacent focalpoints are spaced apart in the plane of the substrate.

Preferably, the distance between the modifications to be producedadjacently in the substrate along the separating line is selected insuch a way that the modified regions are directly mutually adjacent orhave a very small distance between them.

The modifications may be produced by laser machining in whichpositioning of the laser machining head and the laser machiningalternate. However, constant relative movement between the laser beam orlaser machining head and the substrate is preferably carried out whilethe laser radiation is deflected onto the substrate, in such a way thatthe laser beam is continuously guided in a “floating” movement over thesubstrate, in such a way that an interrupted change in the relativeposition results in an extremely rapid machining duration. Inparticular, the relative position of the substrate with respect to thelaser beam can be changed at a constant speed, in such a way that for aconstant pulse frequency the spacing of the modifications to be producedadheres to a predetermined grid dimension.

Particularly preferably, the laser is operated at a wavelength to whichthe substrate is transparent, ensuring penetration of the substrate. Inparticular, this ensures a substantially cylindrical modification regioncoaxial with respect to the laser beam axis, which leads to a constantdiameter of the through-opening or recess.

Further, it may also be advantageous if the laser also additionallyremoves a surface region so as to configure the action region in amanner resulting in a conical inlet region to the through-opening. Inthis manner, the subsequent separation process is simplified. Inaddition, the action of an etching agent is concentrated in this region,for example.

In another, also particularly promising embodiment of the method, thesubstrate is coated in a planar manner with an etch resist on at leastone surface prior to the laser treatment. As a result of the action of alaser beam, the etch resist is removed on at least one surface in adot-like action region and the modification is produced in the substratesimultaneously. In this way, the unmodified regions are protectedagainst undesired action in the subsequent etching process, and thesurface is therefore not damaged. The etch resist does not prevent themodification of the substrate positioned below. Rather, the etch resistis either permeable to the laser radiation or it is removed in a neardot-like manner by the laser radiation, that is to say evaporated, forexample. Further, the possibility is not excluded that the etch resistmay contain substances which act to promote the modification, forexample which accelerate the modification process.

Fundamentally, the method is not limited to particular materialcompositions of the substrate. However, it is particularly promising forthe substrate to comprise an aluminosilicate, in particular aboroaluminosilicate, as a significant material proportion.

A defined separating face can be produced along the modified regions, itoptionally being possible to optimize the separation using additionalexternal force action or a thermal after-treatment, in such a way that asubsequent etching method can be rendered superfluous.

Preferably, the material separation is brought about in the modifiedregions of the substrate by anisotropic material removal by liquidetching, dry etching or vapor phase etching, and optionally also byhigh-voltage or high-frequency evaporation. Optionally, the separationprocess may further be promoted by an external force action, inparticular a tensile force or compressive force. Alternatively, theseparation process can also be carried out without difficulty withoutexternal force actions if the substrate is biased under internal stress.

The second object is achieved according to the invention by a devicecomprising a laser machining head for deflecting laser radiation onto asubstrate, in that the device is equipped with a transmissive medium,which in particular is provided with at least one planar face or is, forexample, configured as a planar plate, and which has a higherintensity-dependent refraction index than air, and which is arranged inparticular between the laser machining head and the substrate in such away that the laser radiation can be deflected through the transmissivemedium onto the substrate. As a result, according to the invention theintensity-dependent refractive index of the transmissive medium isexploited so as to produce an axial change in the focal point during theduration of each individual pulse and the accompanying fluctuation inintensity during the individual pulse, in connection with a pulsedlaser. Thus, unlike in the prior art, the focal point is not unchanged,at least during the duration of an individual pulse, but rather thefocal point is displaced along a line on the beam axis with respect tothe total duration of the individual pulse. It is easy to see whatsignificant advantages result in the present invention from the factthat the focal point is displaced without tracking of the focusingoptics of the laser machining head. In particular, this greatly reducesthe machining duration and also the control system outlay. For example,in a planar substrate the tracking of the z-axis can be omitted. Toproduce the desired separating face, a large number of laser pulses areintroduced into the substrate mutually adjacently.

In principle, a variant is also conceivable in which the transmissivemedium is arranged on the laser machining head upstream of focusingoptics thereof in the direction of the beam path, in such a way that thelaser radiation is initially deflected through the transmissive mediumand subsequently through the focusing optics and directed onto thesubstrate.

The effect of the intensity-dependent light refraction can, of course,be adapted to the respective application, for example in that thetransmissive medium is adapted or replaced accordingly or in that thelaser beam passes through a plurality of transmissive media or throughthe same medium repeatedly.

The focal point may be directed onto a rear face of the substrate,remote from the laser machining head, and the transmissive medium may beformed in such a way that the intensity-dependent focal point reaches afront face, facing the laser machining head, at the intensity maximum.However, it is particularly expedient in practice if the laser radiationcan be deflected onto a focal point at a distance from a rear face ofthe substrate, remote from the laser machining head, in such a way thatthe rear face of the substrate is reached during the increasingintensity progression rather than at an intensity minimum. This ensuresa laser radiation intensity within the substrate which is alwayssufficient for the modification which is to be achieved.

In principle, any pulsed laser is suitable for the machining, a laserhaving a pulse duration of less than 50 ps, preferably less than 5 ps,having been found to be particularly expedient.

In addition, it is particularly expedient if, for focusing, the lasermachining head has focusing optics having a numerical aperture (NA)greater than 0.3, in particular greater than 0.4.

A particularly promising embodiment of the device according to theinvention is also achieved in that the focusing optics have a gradientindex lens. As a result of a lens of this type, also known as a GRINlens, the refractive index which decreases in the radial directionresults in the reduction in intensity which otherwise occurs beinggenerally compensated in the edge region of the lens.

It is further found to be advantageous if the transmissive mediumconsists of glass, in particular quartz glass, so as to provide apronounced intensity-dependent refractive index.

In this context, the transmissive medium is preferably connected to thelaser machining head and arranged so as to be movable together therewithand arranged in particular replaceably on the laser machining head.Rapid fixing, for example, is suitable for this purpose.

Preferably, the device is equipped with a continuously emitting laser inaddition to a pulsed laser, the transmissive medium being transparent tothe wavelength of the continuously emitting laser, and the continuouslyemitting laser being directed onto the glass substrate through themedium or directed onto the glass substrate while circumventing thetransmissive medium. The wavelengths of the pulsed laser and of thecontinuously emitting laser may be different. Further, the laserradiation from the different laser sources may be directed onto theglass substrate from different sides.

FIG. 1 is a schematic drawing of a laser machining method also known as“stealth dicing”. As can be seen, in this context the laser beam isdirected onto a special intermediate layer within a substrate, saidlayer being modified by the laser radiation to form the starting pointfor the subsequent separation of the substrate. An external tensilestress subsequently brings about the separation of the substrate intosub-regions along the action points in the layer.

FIG. 2 shows the individual method steps of introducing a plurality ofthrough-openings into an interposer 1, intended as a contacting elementin circuit board production, comprising a substrate 2. For this purpose,laser radiation 3 is directed onto the surface of the substrate 2. Thesubstrate 2 comprises a boroaluminosilicate as a significant materialproportion, so as to ensure thermal expansion similar to that ofsilicon. The material thickness d of the substrate 2 is between 50 μmand 500 μm. The duration of action of the laser radiation 3 is selectedto be extremely short, in such a way that merely a modification of thesubstrate 2 occurs concentrically about a beam axis of the laser beam,without resulting in significant destruction or considerable materialremoval of the substrate material. In particular, the duration of actionis limited to the individual pulse. For this purpose, the laser isoperated at a wavelength to which the substrate 2 is transparent. Aregion 4 modified in this manner is shown in FIG. 2b . In a followingmethod step, shown in FIG. 2c , the modified regions 4 of the substrate2 which have previously undergone modification by the laser radiation 3form a separating face 5 along the linear succession of modified regions4 in the substrate.

The following describes in more detail an important effect during thelaser machining of the substrate 2 with reference to FIGS. 3 and 4. Thisis the intensity-dependent focal point during an individual pulse P. Theinvention is based on the finding that the intensity I of an individualpulse P of the laser radiation 3 is not constant, but rather has anintensity which increases from a minimum I_(a) through an average I_(b)to a maximum I_(c) and subsequently decreases over the temporalprogression of the individual pulse as shown in FIG. 4, for example, inaccordance with a normal distribution. Simultaneously, as a result ofthe variable intensity I, the refractive index, in particular also of atransmissive medium 8, changes in relation to an individual pulse P overthe temporal progression t. As a result, the intensity-dependent focalpoints 9 a, 9 b, 9 c of the laser radiation 3, which are shown in FIGS.3a to 3c , also change independently of the geometric focal pointdetermined by focusing optics of a laser machining head 10. This effectis amplified by the transmissive medium 8, for example made of glass,which is arranged between the laser machining head 10 and the substrate2 and which has a greater intensity-dependent refractive index than air,in such a way that the distance between the focal points 9 a, 9 cbetween a maximum intensity I_(c) and a minimum intensity I_(a) at leastcorresponds to the desired longitudinal extension, in other words to thedepth of the recess to be produced or, if, as shown, a separating face 5is to be produced, to the material thickness d of the substrate 2. Theintensity-dependent focal point 9 a, 9 b, 9 c thus migrates along thebeam axis Z from a position, which is shown in FIG. 3a and is at adistance from a rear face 11 of the substrate 2, in the direction of thelaser machining head 10, and thus reaches all positions along the beamaxis Z between the rear face 11 and a front face 12 facing the lasermachining head 10 in a continuous movement, in such a way that thedesired modification occurs in the region of the entire primaryextension of the recesses which are subsequently to be produced.

Additionally, FIG. 3a shows, merely schematically, an additional lasermachining head 13, which, to supplement a continuously emitting lasersource connected to the laser machining head 10, directs the laserradiation 3 of a pulsed laser onto the glass substrate 2 selectivelythrough or circumventing the transmissive medium 8. As a result, theintensity I, shown in FIG. 4, of an individual pulse P of the laserradiation 3 is accordingly amplified by the intensity of thecontinuously emitting laser source.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive. Itwill be understood that changes and modifications may be made by thoseof ordinary skill within the scope of the following claims. Inparticular, the present invention covers further embodiments with anycombination of features from different embodiments described above andbelow. Additionally, statements made herein characterizing the inventionrefer to an embodiment of the invention and not necessarily allembodiments.

The terms used in the claims should be construed to have the broadestreasonable interpretation consistent with the foregoing description. Forexample, the use of the article “a” or “the” in introducing an elementshould not be interpreted as being exclusive of a plurality of elements.Likewise, the recitation of “or” should be interpreted as beinginclusive, such that the recitation of “A or B” is not exclusive of “Aand B,” unless it is clear from the context or the foregoing descriptionthat only one of A and B is intended. Further, the recitation of “atleast one of A, B, and C” should be interpreted as one or more of agroup of elements consisting of A, B, and C, and should not beinterpreted as requiring at least one of each of the listed elements A,B, and C, regardless of whether A, B, and C are related as categories orotherwise. Moreover, the recitation of “A, B, and/or C” or “at least oneof A, B, or C” should be interpreted as including any singular entityfrom the listed elements, e.g., A, any subset from the listed elements,e.g., A and B, or the entire list of elements A, B, and C.

The invention claimed is:
 1. A method for separating a substrate usingan optical system configured to provide laser radiation, a thickness ofthe substrate not exceeding 2 mm in a region of a separating line, themethod comprising: applying pulsed laser radiation having a pulseduration (t) to a substrate material of the substrate using the opticalsystem, the substrate material being transparent at least in part to alaser wavelength of the pulsed laser radiation, the pulsed laserradiation being focused using the optical system having at an originalfocal depth (f1), an intensity of the pulsed laser radiation leading toa modification of the substrate along a beam axis (Z) of the pulsedlaser radiation, but not to material removal which goes all the waythrough the substrate, and the pulsed laser radiation being moved alongany desired separating line parallel to a primary extension plane of thesubstrate, bringing about a subsequent separation process along theseparating line, wherein the pulsed laser radiation is focused by theoptical system, which is unchanged per se, by non-linear self-focusingwithin the pulse duration (t) of an individual pulse (P) of the pulsedlaser radiation at a focal depth (f2) different from the original focaldepth (f1), and wherein the focal depth (f2) is less than the originalfocal depth (f1).
 2. The method of claim 1, wherein a difference betweenthe focal depth (f2) and the original focal depth (f1) is greater thanthe thickness of the substrate in the region of the separating line tobe produced, but is at least 20 μm.
 3. The method of claim 1, whereinthe pulse duration (t) of the pulsed laser radiation is less than 50 ps.4. The method of claim 1, wherein the substrate comprises glass,sapphire, and/or silicon, as a significant material proportion.
 5. Themethod of claim 1, wherein the subsequent separation process takes placealong the separating line as a result of internal stresses in thesubstrate.
 6. The method of claim 5, wherein the internal stresses arebrought about by an external force action on the substrate.
 7. Themethod of claim 5, wherein the internal stresses are brought about bythermal stresses.
 8. The method of claim 1, wherein the subsequentseparation process takes place by anisotropic removal substantiallyalong the separating line.
 9. The method of claim 8, wherein theanisotropic material removal comprises etching.
 10. The method of claim9, wherein the etching is carried out in hydrofluoric acid.
 11. Themethod of claim 1 further comprising: coating the substrate with an etchresist at least on one side prior to the applying pulsed laserradiation.
 12. The method of claim 11, wherein the substrate is providedon one side with one or more layers of the etch resist, each layerhaving a respective thickness of less than 10 μm.
 13. The method ofclaim 1, further comprising, while the pulsed laser radiation is actingon the substrate: moving the substrate relative to the pulsed laserradiation and/or a laser machining head.
 14. The method of claim 1,wherein the substrate is planar.
 15. The method of claim 3, wherein thepulse duration (t) of the pulsed laser radiation is less than 5 ps.